The capture of radiation-detection views of a given object using penetrating energy (such as X-rays or the like) is well known in the art. Such radiation-detection views often comprise images having areas that are relatively darker or lighter (or which otherwise contrast with respect to one another) as a function of the density, path length, and composition of the constituent materials that comprise the object being imaged. This, in turn, can serve to provide views of objects that are otherwise occluded from visual inspection.
The use of radiation-detection views finds myriad applications. In at least some application settings, however, merely ascertaining the presence or shape of an occluded object may be insufficient to address all attendant needs. In a security application setting, for example, objects that pose a serious security concern may share a same shape with other completely innocuous objects. In cases where the densities of such objects are similar, it can become impossible to discern from such data which constitutes a threat and which does not. A similar problem can occur when the density and path length product for two objects is substantially the same notwithstanding that they are formed of different materials. As a simple illustration in this regard, a four inch by four inch by three inch block of steel may look the same using one-dimensional radiography as a four inch by four inch by 1.75 inch block of lead notwithstanding that these two materials have considerably different densities.
It is also known in the art to employ two radiation-detection views of a same object formed using two different source-spectrum beams. In particular, one of the source-spectrum beams has a higher typical energy than the other. By comparing the differing energy attenuation information gleaned from such views, one can obtain additional information that relates to the composition of the object being viewed. In particular, the attenuation coefficients will vary with the utilized energy in a manner that depends on the chemical composition of the object.
Such a dual-energy approach can provide satisfactory results in some limited application settings. Generally speaking, when using relatively lower energies (as when the highest utilized energy does not exceed 1.022 MeV) the variation in attenuation results depends mainly upon differing coherent scattering, photoelectric behaviors, and Compton effects. At higher energies, however, the applicant notes that pair-production phenomena play an increasingly important role. In particular, pair production-influenced attenuation increases with increasing atomic number of the material being considered and increases with increasing photon energy while the attenuation due to other processes tends to either increase with increasing atomic number or remain substantially invariant to atomic number, but decreases with increasing photon energy. Therefore, for a given atomic number, there exists some energy (and this energy is above 1.022 MeV) where the attenuation stops decreasing with increasing energy and starts to increase, and the photon energy at which this change occurs is a function of atomic number.
In the dual-energy approach, one can in general glean information on the composition of an object by scanning with two different spectra and comparing the values measured using each spectrum. For example, using energies below 1.022 MeV, the ratio of low-energy transmission to high-energy transmission generally decreases with increasing atomic number, whereas using energies all above 10 MV, the ratio of low-energy transmission to high-energy transmission generally increases with increasing atomic number. For energies in between, generally the ratio of low-energy transmission to high-energy transmission increases with increasing atomic number up to a certain point, then the ratio begins to decrease with further increases to atomic number. Accordingly, a particular relatively high-Z material (such as Uranium) can have the same ratio as some lower-Z material. This leads to a corresponding ambiguity regarding the identity of the material. As a result, some materials cannot be reliably discriminated from one another when using high energies with only two spectra. Unfortunately, there are numerous application settings where high energies must be utilized.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.